U.S. patent number 6,618,120 [Application Number 09/976,303] was granted by the patent office on 2003-09-09 for devices and methods for compensating for tilting of a leveling table in a microlithography apparatus.
This patent grant is currently assigned to Nikon Corporation. Invention is credited to Toshio Ueta.
United States Patent |
6,618,120 |
Ueta |
September 9, 2003 |
**Please see images for:
( Certificate of Correction ) ** |
Devices and methods for compensating for tilting of a leveling
table in a microlithography apparatus
Abstract
With respect to exposure apparatus, apparatus and methods are
disclosed for compensating for lateral shift of a leveling table
caused by a tilt (.theta.) of the leveling table. One embodiment
includes a wafer-stage-position loop servo, a leveling-table
tilt-position loop servo, and a first feed-forward loop from the
leveling-table tilt-position loop servo to the wafer-stage-position
loop servo. The first feed-forward loop converts a torque-control
signal for the leveling table to a linear-acceleration-control
signal for the wafer stage. Thus, the wafer stage moves laterally
to compensate for lateral shift of the leveling table. If the
exposure apparatus includes a reticle stage controlled by a
reticle-stage-position loop servo, then the subject apparatus can
include (in addition to or in place of the first feed-forward loop)
a second feed-forward loop from the leveling-table tilt-position
loop servo to the reticle-stage-position loop servo. The second
feed-forward loop converts a positional signal for the leveling
table to a position-control signal for the reticle stage, to cause
the reticle stage to undergo lateral compensatory motion. The
methods and apparatus provide a faster and more accurate
compensation for lateral shifts of the leveling table.
Inventors: |
Ueta; Toshio (Foster City,
CA) |
Assignee: |
Nikon Corporation (Tokyo,
JP)
|
Family
ID: |
25523967 |
Appl.
No.: |
09/976,303 |
Filed: |
October 11, 2001 |
Current U.S.
Class: |
355/72 |
Current CPC
Class: |
G03F
7/70725 (20130101); G03F 9/7034 (20130101) |
Current International
Class: |
G03F
7/20 (20060101); G03F 9/00 (20060101); G03B
027/58 () |
Field of
Search: |
;355/72,53,39 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Adams; Russell
Assistant Examiner: Esplin; D. Ben
Attorney, Agent or Firm: Klarquist Sparkman LLP
Claims
What is claimed is:
1. In an exposure apparatus including a wafer stage movable at
least in X- and Y-directions and a leveling table tiltable relative
to a Z-axis that is perpendicular to the X- and Y-directions, a
control system hat compensates for lateral shift of the leveling
table caused by a tilt (.theta.) of the leveling table, the control
system comprising: a wafer-stage-position loop servo configured to
actuate movement of the wafer stage in response to a positional
command; a leveling-table tilt-position loop servo configured to
apply a tilting torque to the leveling table in response to a
leveling command; and a feed-forward loop from the leveling-table
tilt-position loop servo to the wafer-stage-position loop servo,
the feed-forward loop being (a) configured to convert a
torque-control signal for the leveling table to a
linear-acceleration-control signal for the wafer stage, the
linear-acceleration-control signal causing the wafer stage to move
laterally in a manner that compensates for the lateral shift of the
leveling table accompanying a change in tilt of the leveling table;
and (b) represented y a block diagram comprising: a coordinate
converter connected to the leveling-table tilt-position loop servo
and configured to convert an angular-acceleration (.theta.")
command from the leveling-table tilt-position loop servo to a
corresponding linear-acceleration command output from the
coordinate converter, a controller connected downstream of the
coordinate converter and configured to apply a factor m.sub.s to
the linear-acceleration command output from the coordinate
converter, wherein m.sub.s is a combined mass of the wafer stage
and leveling table, thereby producing a first force command; and an
adder situated and configured to add the first force command to a
second force command produced by the wafer-stage-position loop
servo, so as to produce an output routed to a wafer-stage
actuator.
2. The control system of claim 1, wherein the exposure apparatus
further includes a reticle stage movable at least in the X- and
Y-directions, and a reticle-stage-position loop servo configured to
actuate movement of the reticle stage in response to a
reticle-position command, the control system further comprising a
second feed-forward loop from the leveling-table tilt-position loop
servo to the reticle-stage-position loop servo, the second
feed-forward loop being configured to convert a positional signal
for the leveling table to a position-control signal for the reticle
stage, the position-control signal causing the reticle stage to
move laterally to compensate, at least in part, for the lateral
shift of the leveling table accompanying a change in tilt of the
leveling table.
3. In an exposure apparatus including a wafer stage movable at
least in X- and Y-directions, a leveling table tiltable relative to
a Z-axis that is perpendicular to the X-and Y-directions, and a
reticle stage movable at least in the X- and Y-directions, a
control system for compensating for lateral shift of the leveling
table caused by a tilt (.theta.) of the leveling table, the control
system comprising: a reticle-stage-position loop servo configured
to actuate movement of the reticle stage in response to a
reticle-position command; a leveling-table tilt-position loop servo
configured to apply a tilting torque to the leveling table in
response to a leveling command; and a first feed-forward loop from
the leveling-table tilt-position loop servo to the
reticle-stage-position loop servo, the first feed-forward loop
being configured to convert a torque-control signal for the
leveling table to a linear-acceleration-control signal for the
reticle stage, the linear-acceleration-control signal causing the
reticle stage to move laterally to compensate for the lateral shift
of the leveling table accompanying a change in tilt of the leveling
table.
4. The control system of claim 3, wherein the first feed-forward
loop is represented by a block diagram comprising: a controller
connected to the leveling-table tilt-position loop servo and
configured to convert a torque command from the leveling-table
tilt-position loop servo to a corresponding linear-acceleration or
force command output from the controller; and an adder situated and
configured to add the command output from the controller to a force
command produced by the reticle-stage-position loop servo, so as to
produce an output routed to a reticle-stage actuator.
5. The control system of claim 4, further comprising: a
wafer-stage-position loop servo configured to actuate movement of
the wafer stage in response to a wafer-position command; and a
second feed-forward loop from the leveling-table tilt-position loop
servo to the wafer-stage-position loop servo, the second
feed-forward loop being configured to convert a torque-control
signal for the leveling table to a linear-acceleration-control
signal for the wafer stage, the linear-acceleration-control signal
causing the wafer stage to move laterally in a manner that
compensates, at least in part, for the lateral shift of the
leveling table accompanying a change in tilt of the leveling table,
with remaining compensation being contributed by the reticle stage
as controlled by the first feed-forward loop.
6. The control system of claim 5, further comprising a
leveling-table tilt sensor located within a feedback loop of the
leveling-table tilt-position loop servo, wherein the second
feed-forward loop is represented by a block diagram comprising a
second controller and a second adder, the second controller being
situated and configured to receive a .theta. output signal from the
leveling-table tilt sensor and to convert the .theta. output signal
to a corresponding positional signal routed to the second adder,
the second adder being configured to add the .theta. output signal
to a wafer-stage-position command routed to the
wafer-stage-position loop servo.
7. The control system of claim 3, further comprising a
wafer-stage-position loop servo, a second feed-forward loop, and a
leveling-table-tilt sensor located within a feedback loop of the
leveling-table tilt-position loop servo, wherein the second
feed-forward loop is represented by a block diagram comprising a
controller and an adder, the controller being situated and
configured to receive a .theta. output signal from the
leveling-table tilt sensor and to convert the .theta. output signal
to a corresponding positional signal routed to the adder, the adder
being configured to add the .theta. output signal to a
wafer-stage-position command routed to the wafer-stage-position
loop servo.
8. An exposure apparatus, comprising: a projection-optical system
having an optical axis parallel to a Z-axis; a wafer stage situated
downstream of the projection-optical system and being movable at
least in the X- and Y-axis directions; a leveling table situated do
stream of the projection-optical system and being tiltable relative
to the Z-axis; a wafer-stage-position loop servo connected to the
wafer stage and configured to actuate movement of the wafer stage
in response to a wafer-position command; a leveling-table
tilt-position loop servo connected to the leveling table and
configured to apply a tilting torque to the leveling table in
response to a leveling command; and a control system for
compensating for lateral shift of the leveling table caused by a
tilt (.theta.) of the leveling table, the control system comprising
a feed-forward loop from the leveling-table tilt-position loop
servo to the wafer-stage-position loop servo, the feed-forward loop
being (a) configured to convert a torque-control signal for the
leveling table to a linear-acceleration-control signal for the
wafer-stage, the linear-acceleration-control signal causing the
wafer stage to move laterally in a manner that compensates for the
lateral shift of the leveling table accompanying a change in tilt
of the leveling table; and (b) represented by a block diagram
comprising: a coordinate converter connected to the leveling-table
tilt-position loop servo and configured to convert an
angular-acceleration (.theta.") command from the leveling-table
tilt-position loop servo to a corresponding linear-acceleration
command output from the coordinate converter; a controller
connected downstream of the coordinate converter and configured to
apply a factor m.sub.s to the linear-acceleration command output of
the coordinate converter, wherein m.sub.s is a combined mass of the
wafer stage and leveling table, thereby producing a first force
command; and an adder situated and configured to add the first
force command to a second force command produced by the
wafer-stage-position loop servo, so as to produce an output routed
to a wafer-stage actuator.
9. The exposure apparatus of claim 8, further comprising: a reticle
stage situated upstream of the projection-optical system and being
movable at least in the X- and Y-directions; and a
reticle-stage-position loop servo connected to the reticle stage
and being configured to actuate movement of the reticle stage in
response to a reticle-position command, wherein the control system
further comprises a second feed-forward loop from the
leveling-table tilt-position loop servo to the
reticle-stage-position loop servo, the second feed-forward loop
being configured to convert a positional signal for the leveling
table to a position-control signal for the reticle stage, the
position-control signal causing the reticle stage to move laterally
to compensate, at least in part, for the lateral shift of the
leveling table accompanying a change in tilt of the leveling
table.
10. An exposure apparatus, comprising: a projection-optical system
having an optical axis parallel to a Z-axis; a leveling table
situated downstream of the projection-optical system and being
tiltable relative to the Z-axis; a reticle stage situated upstream
of the projection-optical system and being movable at least in X-
and Y-directions; a leveling-table tilt-position loop servo
connected to the leveling table and being configured to apply a
tilting torque to the leveling table in response to a leveling
command; a reticle-stage-position loop servo connected to the
reticle stage and being configured to actuate movement of the
reticle stage in response to a reticle-position command; and a
control system for compensating for lateral shift of the leveling
table caused by a tilt (.theta.) of the leveling table, the control
system comprising a first feed-forward loop from the leveling-table
tilt-position loop servo to the reticle-stage-position loop servo,
the first feed-forward loop being configured to convert a
torque-control signal for the leveling table to a
linear-acceleration-control signal for the reticle stage, the
linear-acceleration-control signal causing the reticle stage to
move laterally to compensate for the lateral shift of the leveling
table accompanying a change in tilt of the leveling table.
11. The exposure apparatus of claim 10, further comprising a
reticle-stage actuator that is (i) situated and configured to move
the reticle stage in response to the reticle-position command, and
(ii) connected to the first feed-forward loop and configured to
move the reticle stage in response to linear-acceleration-control
signal.
12. The exposure apparatus of claim 10, wherein the first
feed-forward loop is represented by a block diagram comprising: a
controller connected to the leveling-table tilt-position loop servo
and configured to convert a torque command from the leveling-table
tilt-position loop servo to a corresponding linear-acceleration
command or force command output from the controller; and an adder
situated and configured to add the command output from the
controller to a force command produced by the
reticle-stage-position loop servo, so as to produce an output
routed to a reticle-stage actuator.
13. The exposure apparatus of claim 10, further comprising: a wafer
stage situated downstream of the projection-optical system and
configured to be movable at least in the X- and Y-axis directions;
and a wafer-stage-position loop servo connected to the wafer stage
and configured to actuate movement of the wafer stage in response
to a wafer-position command.
14. The exposure apparatus of claim 13, wherein the control system
further comprises a second feed-forward loop from the
leveling-table tilt-position loop servo to the wafer-stage-position
loop servo, the second feed-forward loop being configured to
convert a torque-control signal to the leveling table to a
linear-acceleration-control signal for the wafer stage, the
linear-acceleration-control signal causing the wafer stage to move
laterally in a manner that compensates, at least in part, for the
lateral shift of the leveling table accompanying a change in tilt
of the leveling table, with remaining compensation being
contributed by the reticle stage as controlled by the first
feed-forward loop.
15. The exposure apparatus of claim 14, further comprising a
leveling-table tilt sensor located within a feedback loop of the
leveling-table tilt-position loop servo, wherein the second
feed-forward loop is represented by a block diagram comprising a
second controller and a second adder, the second controller being
situated and configured to receive a .theta. output signal from the
leveling-table tilt sensor and to convert the .theta. output signal
to a corresponding positional signal routed to the second adder,
and the second adder being configured to add the .theta. output
signal to a wafer-stage-position command routed to the
wafer-stage-position loop servo.
16. In an exposure method in which a substrate, mounted on a wafer
stage movable at least in mutually perpendicular X- and Y-axis
directions, is exposed to a pattern defined by a reticle, a method
for maintaining an alignment of the substrate for exposure, the
method comprising: mounting the substrate on leveling table that is
tiltable relative to a Z-axis perpendicular to the X- and Y-axis
directions, the leveling table being mounted on the wafer stage and
being controlled by a leveling-table tilt-position loop servo that
applies a tilting torque to the leveling table as required in
response to a leveling command corresponding to a torque-control
signal, and the wafer stage being controlled by a
wafer-stage-position loop servo that actuates movement of the wafer
stage at least in the X- and Y-axis directions as required in
response to a wafer-position command; in association with tilting
of the leveling table in response to the tilting command, feeding
forward the torque-control signal from the leveling-table
tilt-position loop servo to the wafer-stage-position loop servo
such that the torque-control signal is converted to an
acceleration-control signal for the wafer stage, the
acceleration-control signal causing the wafer stage to move
laterally to compensate for a lateral shift of the leveling table
caused by tilting of the leveling table; mounting the reticle in a
reticle stage movable at least in the X- and Y-axis directions and
controlled by a reticle-stage-position loop servo that actuates
movement of the reticle stage in response to a reticle-position
command; and in association with tilting of the leveling table in
response to the tilting command, (i) feeding forward the
torque-control signal from the leveling-table tilt-position loop
servo to the reticle-stage-position loop servo, (ii) converting the
fed-forward torque-control signal to a linear-acceleration-control
signal for the reticle stage, and (iii) applying the
linear-acceleration-control signal to the reticle stage to cause
the reticle stage to move laterally to compensate for a lateral
shift of the leveling table accompanying a change in tilt of the
leveling table.
17. In an exposure method in which a substrate, mounted on a wafer
stage movable at least in mutually perpendicular X- and Y-axis
directions, is exposed to a pattern defined by a reticle mounted on
a reticle stage, a method for maintaining an alignment of the
substrate for exposure, the method comprising: mounting the
substrate on a leveling table that is tiltable relative to a Z-axis
that is perpendicular to the X- and Y-axis directions, the leveling
table being mounted on the wafer stage and being controlled by a
leveling-table tilt-position loop servo that applies a tilting
torque to the leveling table as required in response to a leveling
command corresponding to a torque-control signal; mounting the
reticle in a reticle stage movable at least in the X- and Y-axis
directions and controlled by a reticle-stage-position loop servo
that actuates movement of the reticle stage in response to a
reticle-position command; and in association with tilting of the
leveling table in response to the tilting command, (i) feeding
forward the torque-control signal from the leveling-table
tilt-position loop servo toward the reticle-stage-position loop
servo, (ii) converting the fed-forward torque-control signal to an
acceleration-control signal for the reticle stage, and (iii)
applying the acceleration-control signal to the reticle stage to
cause the reticle stage to move laterally to compensate for a
lateral shift of the leveling table accompanying a change in tilt
of the leveling table.
18. The method of claim 17, including the step, while causing the
reticle stage to move laterally to compensate for the lateral shift
of the leveling table, of inhibiting compensating motions of the
wafer stage.
19. A method for operating an exposure apparatus, comprising the
method for maintaining alignment as recited in claim 16.
20. A method for making an object, the method comprising a
microlithography process that includes the method for operating an
exposure apparatus as recited in claim 19.
21. A method for processing a wafer, comprising the method of
operating an exposure apparatus as recited in claim 19.
22. A method for operating an exposure apparatus, comprising the
method for maintaining alignment as recited in claim 17.
23. A method for making an object, the method comprising a
lithography process that includes the method for operating an
exposure apparatus as recited in claim 22.
24. A method for processing a wafer, comprising the method of
operating an exposure apparatus as recited in claim 22.
25. A positioning apparatus for positioning a substrate,
comprising: a first stage movable at least in a first direction; a
second stage mounted on the first stage, the second stage being
configured to retain a substrate and being tiltable relative to the
first stage; and a control system connected to the first stage and
the second stage, the control system comprising a
first-stage-position loop for the first stage, a
second-stage-position loop for the second stage, and a feed-forward
loop connected to the first-stage-position loop and the
second-stage-position loop; wherein the first-stage-position loop
actuates movement of the first stage by utilizing a first-stage
control signal; the second-stage-position loop actuates a tilting
motion of the second stage by utilizing a second-stage control
signal; the feed-forward loop converts the second-stage control
signal to the first-stage control signal; and the first-stage
control signal causes the first stage to move in a manner that
compensates for the lateral shift of the substrate accompanying a
change in tilt of the second stage.
26. A positioning apparatus that positions a substrate, comprising:
a first stage that is movable at least in a first direction; a
second stage mounted on the first stage, the second stage being
configured to retain the substrate and be tiltable relative to the
first stage; a third stage that is movable at least in the first
direction, the third stage being configured to move synchronously
with the substrate moved by the first stage; and a control system
connected to the first, second, and third stages, the control
system comprising a second-stage-position loop for the second
stage, a third-stage-position loop for the third stage, and a first
feed-forward loop connected to the second-stage-position loop and
the third-stage-position loop; wherein the second-stage-position
loop actuates a tilting motion of the second stage by utilizing a
second-stage control signal; the third-stage-position loop actuates
movement of the third stage by utilizing a third-stage control
signal; the first feed-forward loop converts the second-stage
control signal to the third-stage control signal; and the
third-stage control signal causes the third stage to move in a
manner that compensates for alignment errors between the third
stage and the substrate accompanying a change in tilt of the second
stage.
27. The positioning apparatus of claim 26, wherein: the control
system further comprises a first-stage-position loop for the first
stage and a second feed-forward loop connected to the
second-stage-position loop and the first-stage-position loop; the
first-stage-position loop actuates movement of the first stage by
utilizing a first-stage control signal; and the second feed-forward
loop converts the second-stage control signal to the first-stage
control signal, the first-stage control signal causing the first
stage to move in a manner that compensates, at least in part, for
alignment errors between the third stage and the substrate
accompanying a change in tilt of the second stage, with remaining
compensation being contributed by the third stage as controlled by
the first feed-forward loop.
28. An exposure apparatus, comprising: an illumination system that
irradiates radiant energy; and the positioning apparatus of claim
26, the positioning apparatus disposing the substrate within a
trajectory of the radiant energy.
29. In an exposure apparatus including a reticle stage movable at
least in X- and Y-directions, a wafer stage movable at least in X-
and Y-directions, a leveling table associated with the wafer stage
and tiltable relative to a Z-axis that is perpendicular to the X-
and Y-directions, and a control system that compensates for lateral
shift of the leveling table caused by a tilt (.theta.) of the
leveling table, the control system comprising: a
wafer-stage-position loop servo configured to actuate movement of
the wafer stage in response to a positional command; a
leveling-table tilt-position loop servo configured to apply a
tilting torque to the leveling table in response to a leveling
command; a feed-forward loop from he leveling-table tilt-position
loop servo to the wafer-stage-position loop servo, the feed-forward
loop being configured to convert a torque-control signal for the
leveling table to a linear-acceleration-control signal for the
wafer stage, the linear-acceleration-control signal causing the
wafer stage to move laterally in a manner that compensates for the
lateral shift of the leveling table accompanying a change in tilt
of the leveling table; and a reticle-stage-position loop servo
configured to actuate movement of the reticle stage in response to
a reticle-position command, the control system further comprising a
second feed-forward loop from the leveling-table tilt-position loop
servo to the reticle-stage-position loop servo, the second
feed-forward loop being configured to convert a positional signal
for the leveling table to a position-control signal for the reticle
stage, the position-control signal causing the reticle stage to
move laterally to compensate, at least in part, for the lateral
shift of the leveling table accompanying a change in tilt of the
leveling table.
30. An exposure apparatus, comprising: a projection-optical system
having an optical axis parallel to a Z-axis; a wafer stage situated
do stream of the projection-optical system and being movable at
least in the X- and Y-axis directions; a leveling table situated
downstream of the projection-optical system and being tiltable
relative to the Z-axis; a wafer-stage-position loop servo connected
to the wafer stage and configured to actuate movement of the wafer
stage in response to a wafer-position command; a leveling-table
tilt-position loop servo connected to the leveling table and
configured to apply a tilting torque to the leveling table in
response to a leveling command; a control system for compensating
for lateral shift of the leveling table caused by a tilt (.theta.)
of the leveling table, the control system comprising a feed-forward
loop from the leveling-table tilt-position loop servo to the
wafer-stage-position loop servo, the feed-forward loop being
configured to convert a torque-control signal for the leveling
table to a linear-acceleration-control signal for the wafer-stage,
the linear-acceleration-control signal causing the wafer stage to
move laterally in a manner that compensates for the lateral shift
of the leveling table accompanying a change in tilt of the leveling
table; a reticle stage situated upstream of the projection-optical
system and being movable at least in the X- and Y-directions; a
reticle-stage-position loop servo connected to the reticle stage
and being configured to actuate movement of the reticle stage in
response to a reticle-position command, wherein the control system
further comprises second feed-forward loop from the leveling-table
tilt-position loop servo to the reticle-stage-position loop servo,
the second feed-forward loop being configured to convert a
positional signal for the leveling table to a position-control
signal for the reticle stage, the position-control signal causing
the reticle stage to move laterally to compensate, at least in
part, for the lateral shift of the leveling table accompanying a
change in tilt of the leveling table.
Description
FILED OF THE INVENTION
This invention pertains to microlithography, which involves the
transfer of a pattern, usually defined by a reticle or mask, onto a
"sensitive" substrate using an energy beam. Microlithography is a
key technique used in the manufacture of microelectronic devices
such as integrated circuits, displays, thin-film magnetic heads,
and micromachines. More specifically, the invention pertains to
methods and devices, used in the context of a microlithography
method and apparatus, respectively, for compensating for lateral
shift accompanying a leveling tilt imparted to a leveling table,
such as a wafer table, associated with the wafer stage.
BACKGROUND OF THE INVENTION
As the density and miniaturization of microelectronic devices has
continued to increase, the accuracy and resolution demands imposed
on microlithographic methods and apparatus also have increased.
Currently, most microlithography is performed using, as an energy
beam, a light beam (typically deep UV light) produced by a
high-pressure mercury lamp or excimer laser, for example. Emerging
microlithographic technologies include charged-particle-beam
("CPB"; e.g., electron-beam) microlithography and "soft-X-ray" (or
"extreme UV") microlithography.
All microlithographic technologies involve pattern transfer to a
suitable substrate, which can be, for example, a semiconductor
wafer (e.g., silicon wafer), glass plate, or the like. So as to be
imprintable with the pattern, the substrate typically is coated
with a "resist" that is sensitive to exposure, in an image-forming
way, by the energy beam in a manner analogous to a photographic
exposure. Hence, a substrate prepared for microlithographic
exposure is termed a "sensitive" substrate.
Microlithography conventionally is performed using any of various
basic approaches including "direct writing," "contact printing,"
and "projection" microlithgraphy. Projection microlithography is
the most common.
Basic aspects of a modern microlithography apparatus ("exposure
apparatus") 10 are shown in FIG. 18, in the context of a
projection-exposure apparatus. A pattern is defined on a reticle
(sometimes termed a "mask") 12 mounted on a reticle stage 14. The
reticle 12 is "illuminated" by an energy beam (e.g., UV light,
charged particle beam, X-rays) produced by a source 16 and passed
through an illumination-optical system 18. As the energy beam
passes through the reticle 12, the beam acquires an ability to form
an image, of the illuminated portion of the reticle 12, downstream
of the reticle 12. The beam passes through a projection-optical
system 20 that focuses the beam on a sensitive surface of a
substrate 22 held on a substrate stage ("wafer stage" or "wafer XY
stage") 24. As shown in the figure, the source 16,
illumination-optical system 18, reticle stage 14,
projection-optical system 20, and wafer stage 24 generally are
situated relative to each other along an optical axis AX. The
reticle stage 14 is movable at least in the X- and .theta..sub.3
-directions via a stage actuator 26 (e.g., linear motor), and the
positions of the reticle stage 14 in the X- and Y-directions are
detected by respective interferometers 28. The apparatus 10 is
controlled by a controller (computer) 30.
The substrate 22 (also termed a "wafer") is mounted on the wafer
stage 24 via a wafer chuck 32 and wafer table 34 (also termed a
"leveling table"). The wafer stage 24 not only holds the wafer 22
for exposure (with the resist facing in the upstream direction) but
also provides for controlled movements of the wafer 22 in the X-
and Y-directions as required for exposure and for alignment
purposes. The wafer stage 24 is movable by a suitable wafer-stage
actuator 23 (e.g., linear motor), and positions of the wafer stage
24 in the X- and Y-directions are determined by respective
interferometers 25. The wafer table 34 is used to perform fine
positional adjustments of the wafer chuck 32 (holding the wafer
22), relative to the wafer stage 24, in the X-, Y-, and
Z-directions. Positions of the wafer table 34 in the X- and
Y-directions are determined by respective wafer-stage
interferometers 36.
The wafer chuck 32 is configured to hold the wafer 22 firmly for
exposure and to facilitate presentation of a planar sensitive
surface of the wafer 22 for exposure. The wafer 22 usually is held
to the surface of the wafer chuck 32 by vacuum, although other
techniques such as electrostatic attraction can be employed under
certain conditions. The wafer chuck 32 also facilitates the
conduction of heat away from the wafer 22 that otherwise would
accumulate in the wafer during exposure.
Movements of the wafer table 34 in the Z-direction (optical-axis
direction) and tilts of the wafer table 34 relative to the Z-axis
(optical axis AX) typically are made in order to establish or
restore proper focus of the image, formed by the projection-optical
system 20, on the sensitive surface of the wafer 22. "Focus"
relates to the position of the exposed portion of the wafer 22
relative to the projection-optical system 20. Focus usually is
determined automatically, using an auto-focus (AF) device 38. The
AF device 38 produces data that is routed to the controller 30. If
the focus data produced by the AF device 38 indicates existence of
an out-of-focus condition, then the controller 30 produces a
"leveling command" that is routed to a wafer-table controller 40
connected to individual wafer-table actuators 40a. Energization of
the wafer-table actuators 40a results in movement and/or tilting of
the wafer table 34 serving to restore proper focus.
Details of a conventional scheme for tilting of the wafer table 34
are shown in FIG. 19, which shows the wafer stage 24, wafer-stage
actuator 23, wafer table 34, and interferometer 36. Relative to the
wafer stage 24, the wafer table 34 is supported by the wafer-table
actuators 40a. Normally, three wafer-table actuators 40a are
provided, supporting the wafer table 34 in a tripod manner,
relative to the wafer stage 24, at respective "push points." The
wafer-table actuators 40a can be, for example, piezo-electric
actuators.
FIG. 19 depicts two closed-loop control systems. A first control
system 42 pertains to tilting of the wafer table 34 relative to the
wafer stage 24. A second control system 44 pertains to lateral
(X-Y) positioning of the wafer stage 34. The first control system
42 is diagrammed as including a comparator 45, a controller 46, and
a converter 47. A leveling command from the controller 30
responsive to an AF condition detected by the AF device 38 (FIG.
18) is routed to the comparator 45. The comparator 45 also is
connected to a feedback loop 48 discussed below. The output signal
of the comparator 45 is routed to the controller 46, which
processes the signal according to a respective transfer function
G.sub.WT. The processed signal from the controller 46 is routed to
the converter 47, which converts the processed signal to a torque
command (voltage) applied to the wafer-table actuators 40a. The
resulting energization of the actuators 40a causes the wafer table
34 to tilt relative to the wafer stage 24 and relative to a line
L.sub.AX parallel to the optical axis AX. The resulting angular
rotation of the wafer table 34 is denoted by .theta.. Data
concerning .theta. is fed back from leveling-table-tilt sensors
(not shown) via the feedback loop 48 to the comparator 45.
The second control system 44 includes a comparator 50, a
wafer-stage controller 51, and a converter amplifier 52. A
stage-position command from the controller 30 is routed to the
comparator 50. The comparator 50 is connected to a feedback loop 53
discussed below. The output signal from the comparator 50 is routed
to the controller 51, which processes the signal according to a
respective transfer function G.sub.WS. The processed signal from
the controller 51 is routed to the converter amplifier 52, which
converts the processed signal to a voltage applied to the
wafer-stage actuator 23. The applied voltage causes the wafer-stage
actuator 23 to move a corresponding distance. The feedback loop 53
routes an output signal from the wafer-table interferometer 36 to
the comparator 50.
As shown in FIG. 19, the center of rotation 54 of the wafer table
34 is not at the height of the wafer table 34 but rather is
situated a distance L below the wafer table 34. If the wafer table
34 tilts the angle .theta. in response to the leveling command, the
edge of the wafer table 34 effectively experiences a lateral shift
of .DELTA.x=L sin .theta.. This lateral shift is detected by the
wafer-table interferometer 36, and the corresponding data is fed
back to the comparator 50. As a result of this feedback, the
wafer-stage actuator 23 can make a compensating movement of the
wafer stage 24.
Many lithographic exposures are made in a scanning manner, wherein
the reticle stage 14 and wafer stage 24 undergo synchronous motion
relative to each other as the pattern is being exposed onto the
wafer. Conventional wafer stages are massive (hundred kilograms or
more) and consequently have a relatively slow response time to a
stage-position command. As a result, a wafer stage 24 provided with
a feedback loop 53 as shown in FIG. 19 simply is incapable of
making a sufficiently rapid lateral movement to compensate for a
change in tilt of the wafer table 34, especially for scanning
exposures. As a result, significant inaccuracy is introduced into
the exposure.
SUMMARY OF THE INVENTION
In view of the shortcomings of conventional apparatus as summarized
above, an object of the invention is to provide apparatus and
methods providing improved and more accurate compensations for
lateral displacement of the leveling table (wafer table) arising
during tilting motions of the leveling table.
As used herein, "compensation" is not limited in meaning to a
complete offset of the lateral displacement of the leveling table.
Desirably, the amount of compensation is at least effective in
reducing the lateral displacement to within alignment
specifications. However, any reduction of lateral displacement,
compared to not reducing the lateral displacement at all, falls
within the scope of "compensation."
"Tilt" and "tilting" of the leveling table is any change in angle
(.theta.) of the plane of the leveling table, relative to a line
(Z-direction line parallel to the optical axis), from a previous
angle at which the leveling table was positioned.
A "position-loop servo" is a closed-loop feedback-control system
governing position of a body such as the wafer stage, leveling
table (wafer table), and/or reticle stage. Achieving an actual
position (in the X-, Y-, and/or Z-axis direction, or a
.theta.-direction) is performed by a suitable actuator such as a
linear motor (wafer stage and reticle stage) or tilting mechanism
(leveling table).
To achieve ends as summarized above, a first aspect of the
invention is directed, in the context of exposure apparatus and
methods, to control systems for compensating for lateral shift of
the leveling table caused by a tilt (.theta.) of the leveling
table. The contextual exposure apparatus includes a wafer stage
that is movable at least in mutually perpendicular X- and
Y-directions and the leveling table that is tiltable relative to a
Z-axis (that is perpendicular to the X-and Y-directions). An
embodiment of the control system comprises a wafer-stage-position
loop servo, a leveling-table tilt-position loop servo, and a
feed-forward loop from the leveling-table tilt-position loop servo
to the wafer-stage-position loop servo. The wafer-stage-position
loop servo is configured to actuate movement of the wafer stage in
response to a positional command. The leveling-table tilt-position
loop servo is configured to apply a tilting torque to the leveling
table in response to a leveling command. The feed-forward loop is
configured to convert a torque-control signal for the leveling
table to a linear-acceleration-control signal for the wafer stage.
The linear-acceleration-control signal causes the wafer stage to
move laterally in a manner that compensates for the lateral shift
of the leveling table accompanying a change in tilt of the leveling
table.
In the embodiment summarized above, the feed-forward loop can be
represented by a block diagram that includes a coordinate
converter, a controller, and an adder. The coordinate converter is
connected to the leveling-table tilt-position loop servo and is
configured to convert an angular acceleration (.theta.", or second
derivative of .theta.) command from the leveling-table
tilt-position loop servo to a corresponding linear-acceleration
command output from the coordinate converter. The controller is
connected downstream of the coordinate converter and is configured
to apply a factor m.sub.s to the linear-acceleration command output
from the coordinate converter (wherein m.sub.s is a combined mass
of the wafer stage and leveling table), thereby producing a first
force command. The adder is situated and configured to add the
first force command to a second force command produced by the
wafer-stage-position loop servo, so as to produce an output routed
to a wafer-stage actuator.
The exposure apparatus can further include a reticle stage movable
at least in the X- and Y-directions. The reticle stage is
controlled by a reticle-stage-position loop servo configured to
actuate movement of the reticle stage in response to a
reticle-position command. In this configuration, the control system
can further comprise a second feed-forward loop from the
leveling-table tilt-position loop servo to the
reticle-stage-position loop servo. The second feed-forward loop is
configured to convert a positional signal for the leveling table to
a position-control signal for the reticle stage. The
position-control signal causes the reticle stage to move laterally
to compensate, at least in part, for the lateral shift of the
leveling table accompanying a change in tilt of the leveling
table.
In another embodiment, the exposure apparatus includes a reticle
stage that is movable at least in the X- and Y-directions, as
summarized above. The control system comprises the
reticle-stage-position loop servo and leveling-table tilt-position
loop servo as summarized above. The control system also includes a
first feed-forward loop from the leveling-table tilt-position loop
servo to the reticle-stage-position loop servo. The
reticle-stage-position loop servo is configured to actuate movement
of the reticle stage in response to a reticle-position command. The
leveling-table tilt-position loop servo is configured as summarized
above. The first feed-forward loop is configured to convert a
torque-control signal for the leveling table to a
linear-acceleration-control signal for the reticle stage. The
linear-acceleration-control signal causes the reticle stage to move
laterally to compensate for the lateral shift of the leveling table
accompanying a change in tilt of the leveling table. The first
feed-forward loop can be represented by a block diagram that
includes a controller and an adder. The controller is connected to
the leveling-table tilt-position loop servo and is configured to
convert a torque command from the leveling-table tilt-position loop
servo to a corresponding linear-acceleration or force command
output from the controller. The adder is situated and configured to
add the command output from the controller to a force command
produced by the reticle-stage-position loop servo, so as to produce
an output routed to a reticle-stage actuator to cause compensatory
motion of the reticle stage.
The control system summarized in the previous paragraph also can
include a wafer-stage-position loop servo and a second feed-forward
loop. The wafer-stage-position loop servo is configured, as
summarized above, to actuate movement of the wafer stage in
response to a wafer-position command. The second feed-forward loop
extends from the leveling-table tilt-position loop servo to the
wafer-stage-position loop servo. The second feed-forward loop is
configured to convert a torque-control signal for the leveling
table to a linear-acceleration-control signal for the wafer stage.
The linear-acceleration-control signal causes the wafer stage to
move laterally in a manner that compensates, at least in part, for
the lateral shift of the leveling table accompanying a change in
tilt of the leveling table. Any remaining compensation can be made
by lateral motion of the reticle stage as controlled by the first
feed-forward loop.
The control system summarized in the previous paragraph also can
include a leveling-table tilt sensor located within a feedback loop
of the leveling-table tilt-position loop servo. With such a
configuration, the second feed-forward loop can be represented by a
block diagram that includes a second controller and a second adder.
The second controller is situated and configured to receive a
.theta. output signal from the leveling-table tilt sensor and to
convert the .theta. output signal to a corresponding positional
signal routed to the second adder. The second adder is configured
to add the .theta. output signal to a wafer-stage-position command
routed to the wafer-stage-position loop servo.
According to another aspect of the invention, exposure apparatus
are provided. An embodiment of such an apparatus comprises a
projection-optical system, a wafer stage, a leveling table, a
wafer-stage-position loop servo, a leveling-table tilt-position
loop servo, and a control system. The projection-optical system has
an optical axis parallel to the Z-axis. The wafer stage is situated
downstream of the projection-optical system and is movable at least
in the X- and Y-axis directions. The leveling table is situated
downstream of the projection-optical system and is tiltable
relative to the Z-axis. The wafer-stage-position loop servo is
connected to the wafer stage and is configured to actuate movement
of the wafer stage in response to a wafer-position command. The
leveling-table tilt-position loop servo is connected to the
leveling table and is configured to apply a tilting torque to the
leveling table in response to a leveling command. The control
system compensates for lateral shift of the leveling table caused
by a tilt (.theta.) of the leveling table. The control system
comprises a feed-forward loop from the leveling-table tilt-position
loop servo to the wafer-stage-position loop servo. The feed-forward
loop is configured to convert a torque-control signal for the
leveling table to a linear-acceleration-control signal for the
wafer-stage. The linear-acceleration-control signal causes the
wafer stage to move laterally in a manner that compensates for the
lateral shift of the leveling table accompanying a change in tilt
of the leveling table. The feed-forward loop can be represented by
a block diagram that includes a coordinate converter, a controller,
and an adder, as summarized above.
The exposure apparatus also can include a reticle stage that is
situated upstream of the projection-optical system and that is
movable at least in the X- and Y-directions. A
reticle-stage-position loop servo is connected to the reticle stage
and is configured to actuate movement of the reticle stage in
response to a reticle-position command. With such a configuration
of an exposure apparatus, the control system desirably includes a
second feed-forward loop from the leveling-table tilt-position loop
servo to the reticle-stage-position loop servo. The second
feed-forward loop is configured to convert a positional signal for
the leveling table to a position-control signal for the reticle
stage. The position-control signal causes the reticle stage to move
laterally to compensate, at least in part, for the lateral shift of
the leveling table accompanying a change in tilt of the leveling
table.
Another embodiment of an exposure apparatus according to the
invention comprises a projection-optical system, leveling table,
reticle stage, leveling-table tilt-position loop servo, and
reticle-stage-position loop servo all as summarized above. The
apparatus also includes a control system for compensating for
lateral shift of the leveling table caused by a tilt (.theta.) of
the leveling table. The control system includes a first
feed-forward loop from the leveling-table tilt-position loop servo
to the reticle-stage-position loop servo. The first feed-forward
loop is configured to convert a torque-control signal for the
leveling table to a linear-acceleration-control signal for the
reticle stage. The linear-acceleration-control signal causes the
reticle stage to move laterally to compensate for the lateral shift
of the leveling table accompanying a change in tilt of the leveling
table.
The apparatus summarized in the preceding paragraph can include a
reticle-stage actuator that is situated and configured to move the
reticle stage in response to the reticle-position command. The
reticle-stage actuator is connected to the first feed-forward loop
and configured to move the reticle stage in response to the
linear-acceleration-control signal to compensate for lateral shift
caused by leveling.
The first feed-forward loop can be represented by a block diagram
that includes a controller and an adder. The controller is
connected to the leveling-table tilt-position loop servo and is
configured to convert a torque command from the leveling-table
tilt-position loop servo to a corresponding linear-acceleration
command or force command output from the controller. The adder is
situated and configured to add the command output from the
controller to a force command produced by the
reticle-stage-position loop servo, so as to produce an output
routed to a reticle-stage actuator.
The apparatus can include a wafer stage and wafer-stage-position
loop servo, as summarized above. In such a configuration, the
control system can further comprise a second feed-forward loop from
the leveling-table tilt-position loop servo to the
wafer-stage-position loop servo. The second feed-forward loop is
configured to convert a torque-control signal to the leveling table
to a linear-acceleration-control signal for the wafer stage. The
linear-acceleration-control signal causes the wafer stage to move
laterally in a manner that compensates, at least in part, for the
lateral shift of the leveling table accompanying a change in tilt
of the leveling table. Any remaining compensation can be made by
lateral motion of the reticle stage as controlled by the first
feed-forward loop.
The apparatus can include a leveling-table-tilt sensor located
within a feedback loop of the leveling-table tilt-position loop
servo. In such a configuration, the second feed-forward loop can be
represented by a block diagram that includes a second controller
and a second adder. The second controller is situated and
configured to receive a .theta. output signal from the
leveling-table tilt sensor and to convert the .theta. output signal
to a corresponding positional signal routed to the second adder.
The second adder is configured to add the .theta. output signal to
a wafer-stage-position command routed to the wafer-stage-position
loop servo.
Yet another aspect of the invention pertains to methods (in the
context of an exposure method in which a substrate, mounted on a
wafer stage, is exposed to a pattern defined by a reticle) for
maintaining an alignment of the substrate for exposure. In an
embodiment of such a method, the substrate is mounted on a leveling
table that is tiltable relative to a Z-axis perpendicular to the X-
and Y-axis directions. The leveling table is mounted on the wafer
stage and is controlled by a leveling-table tilt-position loop
servo that applies a tilting torque to the leveling table as
required in response to a leveling command corresponding to a
torque-control signal. The wafer stage is controlled by a
wafer-stage-position loop servo that actuates movement of the wafer
stage at least in the X- and Y-axis directions as required in
response to a wafer-position command. In association with tilting
of the leveling table in response to the tilting command, a
torque-control signal is fed forward from the leveling-table
tilt-position loop servo to the wafer-stage-position loop servo
such that the torque-control signal is converted to an
acceleration-control signal for the wafer stage. The
acceleration-control signal causes the wafer stage to move
laterally to compensate for a lateral shift of the leveling table
caused by tilting of the leveling table.
The method can include mounting the reticle in a reticle stage
movable at least in the X- and Y-axis directions and controlled by
a reticle-stage-position loop servo that actuates movement of the
reticle stage in response to a reticle-position command. In such an
embodiment, three actions occur in association with tilting of the
leveling table in response to the tilting command: (1) feeding
forward the torque-control signal from the leveling-table
tilt-position loop servo to the reticle-stage-position loop servo;
(2) converting the fed-forward torque-control signal to a
linear-acceleration-control signal for the reticle stage; and (3)
applying the linear-acceleration-control signal to the reticle
stage to cause the reticle stage to move laterally to compensate
for a lateral shift of the leveling table accompanying a change in
tilt of the leveling table.
In another embodiment of a method according to the invention, the
substrate is mounted on a leveling table as summarized above. The
leveling table is mounted on the wafer stage and is controlled by a
leveling-table tilt-position loop servo that applies a tilting
torque to the leveling table as required in response to a leveling
command corresponding to a torque-control signal. The reticle is
mounted on a reticle stage that is movable at least in the X- and
Y-axis directions and is controlled by a reticle-stage-position
loop servo that actuates movement of the reticle stage in response
to a reticle-position command. In association with tilting of the
leveling table in response to the tilting command, three actions
occur: (1) feeding forward the torque-control signal from the
leveling-table tilt-position loop servo toward the
reticle-stage-position loop servo; (2) converting the fed-forward
torque-control signal to an acceleration-control signal for the
reticle stage; and (3) applying the acceleration-control signal to
the reticle stage to cause the reticle stage to move laterally to
compensate for a lateral shift of the leveling table accompanying a
change in tilt of the leveling table.
The method summarized in the preceding paragraph can include the
step, while causing the reticle stage to move laterally to
compensate for the lateral shift of the leveling table, of
inhibiting compensating motions of the wafer stage.
According to another aspect of the invention, positioning apparatus
are provided for positioning a substrate. An embodiment of such an
apparatus includes a first stage movable at least in a first
direction and a second stage mounted on the first stage. The second
stage is configured to retain a substrate and is tiltable relative
to the first stage. The apparatus includes a control system
connected to the first stage and the second stage. The control
system includes a first-stage-position loop for the first stage, a
second-stage-position loop for the second stage, and a feed-forward
loop connected to the first-stage-position loop and the
second-stage-position loop. The first-stage-position loop actuates
movement of the first stage by utilizing a first-stage control
signal. The second-stage-position loop actuates a tilting motion of
the second stage by utilizing a second-stage control signal. The
feed-forward loop converts the second-stage control signal to the
first-stage control signal. The first-stage control signal causes
the first stage to move in a manner that compensates for the
lateral shift of the substrate accompanying a change in tilt of the
second stage.
Another embodiment of a positioning apparatus includes a first
stage and second stage as summarized above. The apparatus also
includes a third stage that is movable at least in the first
direction and configured for motion in a synchronous manner with
the substrate moved by the first stage. A control system is
connected to the first, second, and third stages. The control
system includes a second-stage-position loop for the second stage
and a third-stage-position loop for the third stage. A first
feed-forward loop is connected to the second-stage-position loop
and the third-stage-position loop. The second-stage-position loop
actuates a tilting motion of the second stage by utilizing a
second-stage control signal. The third-stage-position loop actuates
movement of the third stage by utilizing a third-stage control
signal. The first feed-forward loop converts the second-stage
control signal to the third-stage control signal, and the
third-stage control signal causes the third stage to move in a
manner that compensates for alignment errors between the third
stage and the substrate accompanying a change in tilt of the second
stage.
In the embodiment summarized above, the control system can further
include a first-stage-position loop for the first stage and a
second feed-forward loop connected to the second-stage-position
loop and the first-stage-position loop. The first-stage-position
loop actuates movement of the first stage by utilizing a
first-stage control signal, and the second feed-forward loop
converts the second-stage control signal to the first-stage control
signal. The first-stage control signal causes the first stage to
move in a manner that compensates, at least in part, for alignment
errors between the third stage and the substrate accompanying a
change in tilt of the second stage. Remaining compensation is
contributed by the third stage as controlled by the first
feed-forward loop.
The foregoing and additional features and advantages of the
invention will be more readily apparent from the following detailed
description, which proceeds with reference to the accompanying
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic block diagram showing a wafer stage and
tiltable wafer table, with associated control systems including a
feed-forward torque-control signal to the wafer stage to cause the
wafer table to undergo a lateral motion serving to compensate for
lateral shift accompanying a tilt of the wafer stage, as described
in the first representative embodiment.
FIG. 2 is a block diagram of a conventional control system used to
derive the control system as described in the second representative
embodiment.
FIG. 3 is a block diagram of the control system described in the
second representative embodiment, which provides a changing axis of
rotation of the wafer table.
FIG. 4 is a block diagram of the control system of Comparison
Example 1.
FIG. 5 is a block diagram of the control system of Example 1.
FIG. 6 is a plot of uncorrected wafer-stage error (obtained in
Comparison Example 1) and corrected wafer-stage error (obtained in
Example 1).
FIG. 7 is a schematic diagram showing a wafer stage, a tiltable
wafer table, and a reticle stage, with associated control systems
including a feed-forward torque-control signal to the reticle stage
to cause the reticle stage to undergo a lateral motion serving to
compensate for lateral shift accompanying a tilt of the wafer
table, as described in the second representative embodiment.
FIG. 8 is a block diagram of the control system of Example 2.
FIG. 9 is a plot of uncorrected wafer-stage error (obtained in
Comparison Example 2) and corrected wafer-stage error (obtained in
Example 2).
FIG. 10 is a block diagram of the control system of Comparison
Example 3.
FIG. 11 is a plot of results obtained using the control system of
Comparison Example 3, wherein the upper plot is of wafer-table
displacement error accompanying a wafer-table tilt (.theta.), and
the lower plot is of wafer-stage displacement error.
FIG. 12 is a block diagram of the control system of Example 3.
FIG. 13 is a plot of results obtained using the control system of
Example 3.
FIG. 14 is a block diagram of the control system of Example 4.
FIG. 15(A) is a plot of results obtained using the control system
of Example 4, but without reticle-stage feed-forward.
FIG. 15(B) is a plot of results obtained using the control system
of Example 4, but with reticle-stage feed-forward.
FIG. 16 is a flow chart of certain steps in an exemplary process
for manufacturing a microelectronic device, as described in the
seventh representative embodiment.
FIG. 17 is a flow chart of certain details of the microlithographic
exposure step in the process of FIG. 16.
FIG. 18 is an elevational schematic diagram showing certain aspects
of a conventional exposure apparatus.
FIG. 19 is a schematic block diagram of a conventional scheme for
tilting the wafer table shown in FIG. 18 and for using the wafer
stage for making lateral motions to compensate for lateral
displacement of the wafer table caused by tilting.
DETAILED DESCRIPTION
This invention is described below in the context of representative
embodiments that are not intended to be limiting in any way.
First Representative Embodiment
This embodiment is depicted in FIG. 1, in which components that are
similar to corresponding components shown in FIG. 19 have the same
respective reference numerals. The controller 46 produces an output
that is a torque (T) command, wherein T=I.theta.", I is inertia,
.theta. is the tilt angle (in radians) as discussed above, and
.theta." is the second derivative of .theta. (i.e., the angular
acceleration of the wafer table 34 about the center of rotation
54). As described above, this torque command is routed to the
converter 47. This torque command also is routed to a coordinate
converter 60 that converts units of .theta." into x" or y" units
(i.e., acceleration in the X-dimension or Y-dimension,
respectively). In this regard, as shown in FIG. 1, the linear
displacement of the wafer table 34 is .DELTA.x=L sin .theta..
Because any amount of tilt .theta. imparted to the wafer table 34
is extremely small, for practical purposes, x=L.theta.. Hence,
dx/dt=L(d.theta./dt) (i.e., x'=L.theta.') and x"=L.theta.". From
T=I.theta.", .theta."=T/I; hence x"=L(T/I). Thus, the torque
(control) signal, upon passing through the coordinate converter 60,
is converted to a x" signal that is routed to a controller 62
(multiplies input by m.sub.s, wherein m.sub.s is the mass of the
wafer stage plus wafer table). The converter 62 thus produces a
force signal F (wherein F=m.sub.s x") fed-forward to an adder
(summing junction) 64 situated between the wafer-stage controller
51 and the converter 52.
The acceleration feed-forward line 66 provides a more rapid
response (compared to a conventional system) to the wafer stage 24
in compensating for lateral displacement (.DELTA.x and/or .DELTA.y)
caused by tilting of the wafer table 34.
Second Representative Embodiment
This embodiment is directed to establishing a usable relationship
between the angular acceleration of a wafer table 34 and
acceleration (in the X-Y plane) of a wafer stage 24.
The wafer-table command reference is .theta..sub.ref =A sin
.omega.t, wherein A is the tilt amplitude of the wafer table 34 and
.omega. is the angular frequency achievable with the wafer table
34.
Beginning with a block diagram as depicted in FIG. 2 (showing a
conventional control system), the angular acceleration .theta." is
used to reduce the output positional deviation of the wafer stage
24. In FIG. 2, .theta..sub.ref is input to a comparator 70. The
output of the comparator 70 is connected to a controller 71 that
produces, according to a respective transfer function G.sub.L (s),
a torque output (T=I.theta."). The controller output is converted
to a .theta." signal by a converter 72. The .theta." signal is
converted by a first integrator 73 to a .theta." signal, and then
by a second integrator 74 to a .theta. signal (.theta..sub.out).
The .theta..sub.out signal is fed back 75 to the comparator 70. The
error component of L sin .theta..sub.out (e.g., x and/or y) caused
by rotation (.theta..sub.out) of the wafer table 34 is added at an
adder 82 via a converter 76. A wafer-stage-position command
WS.sub.in is input to a comparator 77. The output of the comparator
77 is processed by a controller 78 according to a respective
transfer function G.sub.S (s). Using the positional signal x as an
example, the output from the controller 78 is converted to an x"
signal by a converter 79, and by a first integrator 80 to an x'
signal, and then by a second integrator 81 to an x signal. The x
signal is routed to an adder (summing junction) 82 that also
receives the output fed-forward from the controller 76. The output
from the adder 82 is the wafer-stage output signal (WS.sub.out),
which is fed back 83 to the comparator 77.
In this example, .theta." (angular acceleration) is used to reduce
deviation in the wafer-stage output (WS.sub.out). In this regard,
reference is made to the block diagram of FIG. 3, in which
components that are similar to those shown in FIG. 2 have the same
respective reference numbers. A control system according to the
block diagram of FIG. 3 (corresponding to the embodiment shown in
FIG. 1) provides a changing axis of rotation of the wafer table
(i.e., the center of rotation of the wafer table shifts from an
initial position toward the wafer on the wafer table). The torque
output (T=I.theta.") is converted to a .theta." signal by a
converter 272. The output of the converter 272 is routed to a
converter 84. The .theta." signal is fed-forward through two
converters 84, 85 (multiply input by L and m.sub.s, respectively,
wherein m.sub.s is the mass of the wafer stage plus wafer table) to
an adder (summing junction) 86 that also receives, as an input, the
output from the controller 78.
The factor (L) by which the converter 84 multiplies its input is
determined as follows. For one degree of freedom of motion (e.g.,
X-direction motion) of the wafer stage, and according to a Jacobian
approach, .differential.x/.differential..theta.=L sin .theta.=L cos
.theta.. Hence, x'=(L cos .theta.)(.theta.'). Since
.differential.x'/.differential..theta.'=L cos .theta., x"=(L cos
.theta.)(.theta."). If .theta. is small, then sin .theta..apprxeq.0
and cos .theta..apprxeq.1. Hence, x"=L.theta." (i.e., acceleration
of the wafer stage is L times the angular acceleration of the wafer
table). Therefore, it is determined that the converter 84 applies
the gain L. The converter 85 multiplies the output of the converter
84 by m.sub.s.
The foregoing analysis is based on one degree of freedom of motion
of the wafer stage. For two degrees of freedom, a more complicated
Jacobian matrix is required.
EXAMPLE 1 AND COMPARISON EXAMPLE 1
In this example, positional deviation of a wafer stage is reduced
using target .theta..sub.x and .theta..sub.y torques of the wafer
table as the feed-forward signal for the wafer stage. This example
is directed to the results of considering two degrees of freedom of
motion of the wafer stage. The target .theta..sub.x and
.theta..sub.y torques are converted to respective target angular
accelerations. The target angular accelerations are based on the
inertial principal axis of the wafer table. Motion of the wafer
stage, in contrast, is based on a mechanical axis. Consequently,
the respective angular accelerations are converted (by processing
according to a Jacobian transformation matrix) to respective
angular accelerations based on mechanical acceleration. This
transformation matrix utilizes a Jacobian for two degrees of
freedom.
The comparison example provides data obtained using a conventional
wafer-table mechanism as shown generally in FIG. 19, which is shown
as a more detailed block diagram in FIG. 4. The block diagram of
FIG. 4 is for a wafer table having three degrees of freedom
(.theta..sub.x, .theta..sub.y and Z). Briefly, in FIG. 4, target
values of .theta..sub.x, .theta..sub.y, and Z (based on a
mechanical coordinate system) are processed by a first transfer
matrix 90 that converts these values to respective sensor values
S.sub.1, S.sub.2, S.sub.3. The sensor values, as processed by a
comparator 91, are routed to a second transfer matrix 92
(corresponding to the controller 71 in FIG. 2) that converts the
S1, S2, S3 values to respective coordinates .theta..sub.xp,
.theta..sub.yp, Z.sub.p on the inertial principal axis coordinate
system. The inertia coordinates (representing desired torque
coordinates .theta..sub.xp, .theta..sub.yp and desired Z.sub.p
force) are processed by a comparator 93 and routed to a third
transfer matrix 94. The transfer matrix 94 converts these desired
torque coordinates and force to corresponding "push-point" forces
P.sub.1, P.sub.2, P.sub.3 applied by respective wafer-table
actuators to the wafer table. These push-point forces P.sub.1,
P.sub.2, P.sub.3 are routed to the wafer-table tilt mechanism 95
(corresponding to the converter 72, first integrator 73, and second
integrator 74 in FIG. 2). The resulting tilt position of the wafer
table is sensed by sensors 96 that produce corresponding positional
data S.sub.1, S.sub.2, S.sub.3.
The sensor data S.sub.1, S.sub.2, S.sub.3 are distributed as
follows. First, these data are differentiated by a differential
block 97, thereby producing corresponding sensor "velocity" data.
The sensor velocity data are processed by a velocity servo Jacobian
matrix 98 for feedback as respective .theta..sub.xp ',
.theta..sub.yp ', Z.sub.p ' data to the comparator 93. The sensor
data S.sub.1, S.sub.2, S.sub.3 also are fed back 99 to the
comparator 91. The error component caused by tilting of the wafer
table 34 is added at the adder 82 via a fourth transfer matrix 100
and a transfer function 101. The sensor data S.sub.1, S.sub.2,
S.sub.3 are converted to respective .theta..sub.x, .theta..sub.y, Z
data by the fourth transfer matrix 100 and added at the adder 82 as
lateral displacement information by the transfer function 101
(e.g., converting .theta..sub.y to L sin .theta..sub.y). The adder
82, as discussed above in connection with FIG. 2, also receives the
output from the controller 78 via the converter 79, the first
integrator 80, and the second integrator 81. The output from the
adder 82 provides one degree of freedom for the wafer stage, and
represents the respective wafer-stage interferometer (IF) data.
FIG. 5 is a block diagram of the example. In FIG. 5, components
that are the same as shown in FIG. 4 have the same respective
reference numerals and are not described further. In contrast to
the scheme depicted in FIG. 4, this example includes a feed-forward
loop as described above. The feed-forward loop begins with data
output from the comparator 93 that are routed to and processed by a
fifth transfer matrix 102 that converts the data .theta..sub.xp,
74.sub.yp, Z.sub.p (based on the inertial principal axis coordinate
system) to respective push-point forces. Data regarding the
push-point forces are processed by a sixth transfer matrix 103 to
respective torque data .theta..sub.xp ", .theta..sub.yp ", Z.sub.p.
A seventh transfer matrix 104 converts the torque data
.theta..sub.xp ", .theta..sub.yp ", Z.sub.p to respective
second-derivative data .theta..sub.x ", .theta..sub.y ", Z based on
the mechanical coordinate system. These data .theta..sub.x ",
.theta..sub.y ", Z are converted (e.g., L.theta..sub.x " to X" and
L.theta..sub.y" to Y") by a coordinate controller 201
(corresponding to the converter 84 shown in FIG. 3), multiplied by
the mass m.sub.s of the wafer stage and wafer table by a converter
202 (corresponding to the converter 85 shown in FIG. 3), and routed
to the comparator 86, as described above in connection with FIG.
3.
Exemplary results are shown in FIG. 6. The curve 110 is of
uncorrected wafer-stage error (X-direction) obtained using the
comparison example, and the curve 112 is of corrected wafer-stage
error (X-direction) obtained using the example. Hence, by providing
the wafer stage with a torque feed-forward compensation, the
leveling-shift deviation of the wafer stage is reduced compared to
a conventional wafer stage having no such compensation.
Third Representative Embodiment
In this embodiment, alignment errors occurring at the wafer stage
due to tilting of the wafer table are eliminated by compensatory
motions of a reticle stage, either alone or in combination with
compensatory motions of the wafer stage. With respect to
compensatory motions by the reticle stage, a "reticle fine stage"
can be used. In this regard, reference is made back to FIG. 18,
which depicts a microlithography apparatus including a reticle
stage 14. In many microlithography apparatus, the reticle 12
actually is mounted on a "reticle fine stage" 13 that is mounted to
the reticle stage 14. Whereas, in such a configuration, the reticle
stage 14 is used to make relatively "coarse" changes in reticle
position (e.g., in the X, Y, and .theta..sub.z directions), the
reticle fine stage 13 can be used to make relatively fine
adjustments in the X, Y, and .theta..sub.z directions. The reticle
fine stage 13 moves the reticle 12 relative to the reticle stage
14. The mass of the reticle fine stage 13 moved in either of the X-
and Y-directions is the same.
This embodiment is especially useful for microlithography apparatus
that perform exposures by scanning (e.g., step-and-scan). Such
apparatus generally require higher performance (e.g., faster
response time) in compensating for tilt of the wafer table than
step-and-repeat apparatus.
An advantage of utilizing the reticle fine stage for making
compensatory motions is that, compared to a wafer stage or even a
wafer table, the reticle fine stage is comparatively light in mass.
A wafer stage, in contrast, is quite massive. (E.g., a wafer stage
capable of accommodating a 300-mm diameter wafer typically has a
mass of at least 100 kg. However, this embodiment and other
embodiments according to the invention can be used with a reticle
coarse stage or other type of reticle stage that consists of one
stage not divided into "coarse" and "fine" stages.) Providing a
control-position feedback loop and a power amplifier exhibiting a
sufficiently high-frequency response for such a massive object is
extremely difficult. Its lighter mass allows the reticle fine stage
to exhibit a higher frequency response, when making compensatory
motions to offset alignment errors due to tilting of the wafer
table, than the wafer stage. Also, in contrast to a typical wafer
stage, the mass of the reticle fine stage movable in the
X-direction is equal to the mass of the reticle fine stage movable
in the Y-direction. Consequently, the reticle fine stage exhibits
substantially the same response frequency in both the X-direction
and the Y-direction. With a wafer stage, in contrast, the mass
movable in one of the X- and Y-directions normally is substantially
greater (usually in the X-direction, due to a greater stacked mass
borne by the X-direction movement mechanism) than in the other
direction. Consequently, the response frequency is lower in the
more massive direction than in the less massive direction.
Motions of the wafer stage (to compensate for tilting of the wafer
table) can result in position-alignment errors between the reticle
and the substrate. According to this embodiment, if compensatory
motions are made by both the wafer stage and the reticle stage,
then the reticle stage undergoes a motion to compensate for
position-alignment errors between the reticle and substrate arising
from a lateral positional shift of the wafer stage.
In making compensatory motions of the reticle stage, it is
desirable under some conditions that such motions be made
separately of any motions by the wafer stage. During motion of the
wafer stage, if the reticle stage is actuated to compensate for the
change in position of the wafer stage, stability is difficult to
obtain. This is because, when the reticle stage moves to
reestablish alignment of the reticle with the wafer, the wafer
stage already has moved to another position due to the action of
its positional feedback loop. The reticle stage receives, by a
feed-forward line, data concerning the lateral shift of the wafer
table; however, this does not provide the reticle stage with
information on how far the wafer stage can move to compensate for
the lateral shift.
A block diagram of a control scheme for utilizing the reticle stage
for compensating for tilting motion of the wafer table is shown
generally in FIG. 7, in which components that are similar to
corresponding components shown in FIG. 1 have the same respective
reference numerals. First, similar to the scheme shown in FIG. 1,
sensor data regarding tilt of the wafer table 34 are fed back 48 to
the comparator 45. In addition, this data is fed forward 115 to an
adder (summing junction) 116 via a conversion matrix 117 that
converts angle data to corresponding positional data useful as a
target position for the wafer stage.
The apparatus of FIG. 7 includes a reticle stage 118 actuated by a
linear actuator 119. The position of the reticle stage 118 is
detected by at least one interferometer 120. A
reticle-stage-position command passes through an adder (summing
junction) 121 and a comparator 122 to a controller 123 that
converts the reticle-stage-position command to corresponding force
(V) signals routed to the linear actuator 119 via a converter 124.
For achieving alignment of the reticle stage 118 with the wafer
stage 24, data from the interferometer 36 is routed to an adder 122
via a converter 125 applies a factor M to the wafer-stage x-y data,
wherein 1/M is the demagnification ratio of the projection-optical
system 20. Alignment errors 126 of the reticle stage 118 relative
to the wafer table are determined by a comparator 127 that receives
an input from the interferometer 120 and an input from the
converter 125. Input from the reticle interferometer 120 also is
fed-back to the comparator 122. Finally, according to this
embodiment, data (regarding torque) from the controller 46 are
fed-forward to an adder (summing junction) 128 via a controller 129
that converts torque data to corresponding x", y" or force
data.
According to the scheme shown in FIG. 7, the reticle-stage servo is
configured to receive data regarding wafer-table lateral position,
so as to compensate for lateral-shift errors caused by tilting of
the wafer table 34. Lateral-shift compensation that otherwise would
be performed by the wafer-stage servo is eliminated via the loop
115 including the conversion matrix 117. Of course, the wafer stage
is still free to move to change the exposure-target position on the
wafer or to correct errors not caused by tilting of the wafer table
34.
EXAMPLE 2 AND COMPARISON EXAMPLE 2
The comparison example for this example is the same as discussed
above in connection with FIGS. 5 and 6. FIG. 5 is the block diagram
of the control system used to produce the curve 112 in FIG. 6. The
plot 110, of uncorrected wafer-stage error, was produced by the
control system shown in FIG. 4.
A block diagram of the control system of Example 2 is shown in FIG.
8, in which components that are similar to corresponding components
shown in FIG. 5 have the same reference numerals and are not
discussed further. The block diagram of FIG. 8 includes use of the
reticle stage to reduce the "following error" of the wafer stage.
In other words, the reticle stage follows the wafer stage as the
reference target of the reticle stage. Beginning at the adder 82,
the output of the adder 82 is routed via a converter 160 (which
applies a factor M to its input, wherein 1/M is the demagnification
ratio of the projection lens) to a comparator 161. The output of
the comparator 161 passes through a controller 162 configured to
process incoming data according to one or more transfer functions
163, 164. The output of the controller 162 passes through a
converter 165, as required, to an adder (summing junction) 166. The
adder 166 also receives data, fed-forward from the converter 160,
as processed by at least one transfer function 167, 168. The output
of the adder 166 is routed to a reticle-stage physical system 170
that includes a converter 169 and two integrators 171, 172. The
output from the reticle-stage physical system 170 enters an adder
(summing junction) 173 having a grounded input. In any event, the
output of the adder 173 includes data from the reticle
interferometer. The output data is routed via a converter 174 (that
applies a factor 1/M to its input) to a comparator 175. The output
from the comparator 175 includes data concerning positional errors
between the reticle and the wafer. To such end, the comparator 175
receives, at its other input, data output from the adder 82. Items
190 and 192 are respective on-off switches.
Results obtained from the block diagram of FIG. 8 are shown in FIG.
9, along with the plots 110 and 112 of data as described above in
connection with FIG. 6. The straight-line plot 180 is of the
"following" error of the reticle stage. Hence, it can be seen that
the reticle-stage following function substantially reduces
wafer-table error. (The plots 110 and 112 are the same as shown in
FIG. 6 and discussed above.)
EXAMPLE 3 AND COMPARISON EXAMPLE 3
A control block diagram of the comparison example is shown in FIG.
10, in which components that are similar to corresponding
components shown in FIG. 2 have the same respective reference
numerals. A control system as shown in FIG. 10 was used to produce
the plots shown in FIG. 11. In this conventional scheme, if the
wafer table undergoes a change in tilt, the wafer stage makes a
compensatory lateral shift as required. In the comparison example,
the distance between the wafer surface and the center of rotation
54 is 40 mm. As the wafer table experiences a 10 .mu.rad tilt, the
wafer stage experiences a lateral compensatory movement of 400 nm.
Two curves are shown in FIG. 11. The upper curve 87 is of "output
a" (wafer table lateral shift) in FIG. 10, and the lower curve 88
is of "output b" (wafer-stage position) in FIG. 10. Note the 400-nm
difference between the two curves.
A block diagram of a control scheme according to Example 3 is shown
in FIG. 12. In this control scheme, the wafer-control system
receives data regarding the lateral shift of a tilting wafer table.
But, the wafer stage maintains a target position even if the
wafer-stage interferometer 36 detects a lateral-shift error. To
such end, the control scheme shown in FIG. 12 includes a
"wafer-stage target control" loop in which data regarding
wafer-table lateral shift is fed-forward from the feedback loop 75
via a controller 130 (corresponding to the conversion matrix 117
shown in FIG. 7) to an adder (summing junction) 131 situated
upstream of the comparator 77. The controller 130 converts the data
regarding wafer-table lateral shift to corresponding positional
data useful as a target position for the wafer stage. The adder 131
also receives data regarding the target wafer-stage position, and
the output from the adder 131 is input to the comparator 77. The
resulting curve 105 is shown in FIG. 13, showing that output (b)
(curve 106; wafer-stage position) has been reduced to zero.
Meanwhile, output (a) (curve 105; wafer-table lateral shift)
exhibits the expected 400-nm lateral shift.
To cause the reticle stage to make a lateral motion to compensate
for tilting of the wafer table, reference is made to the block
diagram of FIG. 14, showing a control system 135 for the reticle
stage. (Respective control systems 136, 137 for the wafer table and
wafer stage are also shown, which are similar to the block diagram
shown in FIG. 12). The control system 135 is connected to the
control system 137 via a converter 138 (that applies a factor M to
its input, wherein 1/M is the demagnfication ratio of the
projection-optical system). Data from the converter 138 are routed
to a comparator 139. The output of the comparator 139 is connected
to a controller 140 that is analogous (in terms of processing
incoming data using at least one transfer function) to the
controllers 78, 71. Data from the controller 140 are passed through
a gain amplifier 141 (analogous to amplifier models 142, 143 in the
control systems 136, 137, respectively) to an adder (summing
junction) 144. The output of the adder 144 is routed to a
reticle-stage physical system 146 and an adder (summing junction)
149. The reticle-stage physical system 146 includes a converter 145
(that applies a factor 1/m to its input, wherein m=resting mass of
the reticle stage) and integrators 147, 148. The output (reticle
interferometer) from the adder 149 is fed-forward through a
converter 150 (that applies the factor 1/M to its input) to a
comparator 154 that also receives, as input, data output from the
adder 82. The output of the adder 149 also is fed-back to the
comparator 139. The adder 144 also receives a "reticle-stage
feed-forward" input converted from wafer-table X, Y lateral
direction with .theta. acceleration. The reticle-stage feed-forward
includes a first converter 372 (that produces a .theta." signal,
similarly to the converter 272 shown in FIG. 3), a second converter
152 (that applies the factor M to its input), a controller 151
(L.theta." to X"), and a third converter 153 (that applies the
factor m to its input).
Results obtained using the control system of FIG. 14 are shown in
FIGS. 15(A) and 15(B). The curves shown in FIG. 15(A) are of data
obtained without reticle-stage feed-forward (FIG. 10), and the
curves shown in FIG. 15(B) are of data obtained with the
reticle-stage feed-forward (FIG. 14). Turning first to FIG. 15(A),
curve 155 is the output of the wafer stage interferometer, and
curve 156 represents the response of the reticle stage. In FIG.
15(B), the curve 157 was obtained with the reticle-stage
feed-forward off, and the curve 158 was obtained with the
reticle-stage feed-forward on. The leveling shift error is
attenuated (curve 158) with feed-forward.
Fourth Representative Embodiment
This embodiment is directed generally to a positioning apparatus
for positioning a substrate. The apparatus includes a "first stage"
movable at least in a first direction. This first stage can be the
wafer stage 24 shown in FIG. 18. The apparatus also includes a
"second stage" (e.g., the wafer table 34; FIG. 18) mounted on the
first stage 24. The second stage is configured to retain a
substrate and is tiltable relative to the first stage 24.
The apparatus also includes a "control system" (e.g., the
controller 30; FIG. 18) connected to the first stage 24 and the
second stage 34. The control system 30 has a first-stage-position
loop for the first stage 24 and a second-stage-position loop for
the second stage 34. An exemplary "first-stage-position loop"
includes a combination of the comparator 50, the wafer-stage
controller 51, the converter amplifier 52, and feed-back loop 53
(FIGS. 1 and 7). Another exemplary first-stage-position loop
includes a combination of the comparator 77, controller 78,
converter 79, first integrator 80, second integrator 81, gain
amplifier 143, and feed-back 83 (FIGS. 3, 5, 8, 12, and 14). An
exemplary "second-stage-position loop" includes a combination of
the comparator 45, controller 46, converter 47, and feed-back loop
48 (FIGS. 1 and 7). Another exemplary second-stage-position loop
includes a combination of the comparator 70, controller 71,
converter 72, first and second integrators 73, 74, respectively,
feed-back line 75, first transfer matrix 90, comparator 91, second
transfer matrix 92, third transfer matrix 94, differential block
97, velocity servo Jacobian matrix 98, feed-back line 99, gain
amplifier 142, and feed-back line 75 (FIGS. 3, 5, 8, 12, and 14).
An exemplary "feed-forward loop" includes a combination of the
coordinate converter 60, controller 62, and adder 64 (FIG. 1).
Another exemplary feed-forward loop includes a combination of the
converters 272 and 84, controller 85, adder 86, fifth transfer
matrix 102, sixth transfer matrix 103, seventh transfer matrix 104,
and on/off switch 192 (FIGS. 3, 5, and 8). The feed-forward loop is
connected to the first-stage-position loop and the
second-stage-position loop.
The first-stage-position loop actuates movement of the first stage
by utilizing a first-stage control signal, and the
second-stage-position loop actuates a tilting motion of the second
stage by utilizing a second-stage control signal. The feed-forward
loop converts the second-stage control signal to the first-stage
control signal, the first-stage control signal causes the first
stage to move in a manner that compensates for the lateral shift of
the substrate accompanying a change in tilt of the second
stage.
Fifth Representative Embodiment
This embodiment is directed generally to a positioning apparatus
for positioning a substrate. The apparatus includes a "first stage"
movable at least in a first direction. This first stage can be the
wafer stage 24 shown in FIG. 18 and includes a first-stage-position
loop. The apparatus also includes a "second stage" (e.g., the wafer
table 34; FIG. 18) mounted on the first stage 24. The second stage
includes a second-stage-position loop. A "feed-forward loop"
interconnects the first-stage-position loop and the
second-stage-position loop. The second stage is configured to
retain a substrate and is tiltable relative to the first stage 24.
The apparatus also includes a "third stage" that is movable at
least in the first direction and configured to move synchronously
with the substrate moved by the first stage. The third stage can be
the reticle fine stage 13 or the reticle stage 14 (FIG. 18).
The apparatus also includes a "control system" (e.g., the
controller 30 in FIG. 18) connected to the first, second, and third
stages. The control system includes a second-stage-position loop
for the second stage, a third-stage-position loop for the third
stage, and a first feed-forward loop connected to the
second-stage-position loop and the third-stage-position loop. An
exemplary "second-stage-position loop" includes a combination of
the comparator 45, controller 46, converter 47, and feed-back loop
48 (FIGS. 1 and 7). Another exemplary second-stage-position loop
includes a combination of the comparator 70, controller 71,
converter 72, first and second integrators 73, 74, respectively,
feed-back line 75, first transfer matrix 90, comparator 91, second
transfer matrix 92, third transfer matrix 94, differential block
97, velocity servo Jacobian matrix 98, feed-back line 99, gain
amplifier 142, and feed-back line 75 (FIGS. 3, 5, 8, 12, and 14).
An exemplary "third-stage-position loop" includes a combination of
the adder 121, comparator 122, controller 123, converters 124, 125,
and comparator 127 (FIG. 7). Another exemplary third-stage-position
loop includes a combination of the comparator 161, controller 162,
converter 165, adder 166, transfer functions 167, 168, on/off
switch 190, reticle physical system 170, adder 173, converter 160,
converter 174, and comparator 175 (FIG. 12). Yet another exemplary
third-stage-position loop includes a combination of the comparator
139, controller 140, gain amplifier 141, reticle physical system
146, adder 149, converter 138, converter 150, and comparator 154
(FIG. 14). An exemplary "first feed-forward loop" includes a
combination of the controller 129 and adder 128 (FIG. 7). Another
exemplary first feed-forward loop is a combination of the converter
372, first converter 152, controller 151, second converter 153, and
adder 144 (FIG. 14).
The second-stage-position loop actuates a tilting motion of the
second stage by utilizing a second-stage control signal. The
"second-stage control signal" includes at least one of the
wafer-table leveling command, torque signal T, .theta." (FIGS. 1
and 3); angular acceleration of the wafer table 34, desired torque
coordinates .theta..sub.xp, .theta..sub.yp, and desired Z.sub.p
force (FIG. 5); sensor data, .theta., torque data (FIG. 7);
wafer-table X, Y lateral direction with .theta. acceleration (FIG.
14).
The third-stage-position loop actuates movement of the third stage
by utilizing a third-stage control signal. The "third-stage control
signal" includes at least one of the reticle-stage-position
command, force (V) signals, x", y", force data (FIG. 7); and
reticle-stage feed-forward input (FIG. 14).
The first feed-forward loop converts the second-stage control
signal to the third-stage control signal. The third-stage control
signal causes the third stage to move in a manner that compensates
for alignment errors between the third stage and the substrate
accompanying a change in tilt of the second stage.
The control system can further include a first-stage-position loop
for the first stage and a second feed-forward loop connected to the
second-stage-position loop and the first-stage-position loop. An
exemplary "first-stage-position loop" includes a combination of the
comparator 50, the wafer-stage controller 51, the converter
amplifier 52, and feed-back loop 53 (FIGS. 1 and 7). Another
exemplary first-stage-position loop includes a combination of the
comparator 77, controller 78, converter 79, first integrator 80,
second integrator 81, gain amplifier 143, and feed-back 83 (FIGS.
3, 5, 8, 12, and 14). An exemplary "second feed-forward loop"
includes a combination of the conversion matrix 117, adder 116, and
x and y position (FIG. 7). Another exemplary second feed-forward
loop includes a combination of the controller 130 and adder 131
(FIGS. 12 and 14). The first-stage-position loop actuates movement
of the first stage by utilizing a first-stage control signal. An
exemplary "first-stage control signal" includes at least one of the
wafer-stage-position command, force signal F, and x" (FIG. 1); and
acceleration of the wafer stage 24, X", and Y" (FIG. 5). The second
feed-forward loop converts the second-stage control signal to the
first-stage control signal. The first-stage control signal causes
the first stage to move in a manner that compensates, at least in
part, for alignment errors between the third stage and the
substrate accompanying a change in tilt of the second stage.
Remaining compensation is contributed by the third stage as
controlled by the first feed-forward loop.
Sixth Representative Embodiment
A microlithography apparatus (generally termed an "exposure
apparatus") with which any of the foregoing embodiments can be used
is depicted in FIG. 18, which is discussed above.
The exposure apparatus 10 can be any of various types. For example,
as an alternative to operating in a "step-and-repeat" manner
characteristic of steppers, the exposure apparatus can be a
scanning-type apparatus operable to expose the pattern from the
reticle 12 to the wafer 22 while continuously scanning both the
reticle 12 and wafer 22 in a synchronous manner. During such
scanning, the reticle 12 and wafer 22 are moved synchronously in
opposite directions perpendicular to the optical axis Ax. The
scanning motions are performed by the respective stages 14, 24.
In contrast, a step-and-repeat exposure apparatus performs exposure
only while the reticle 12 and wafer 22 are stationary. If the
exposure apparatus is an "optical lithography" apparatus, the wafer
22 typically is in a constant position relative to the reticle 12
and projection-optical system 20 during exposure of a given pattern
field. After the particular pattern field is exposed, the wafer 22
is moved, perpendicularly to the optical axis Ax and relative to
the reticle 12, to place the next field of the wafer 22 into
position for exposure. In such a manner, images of the reticle
pattern are sequentially exposed onto respective fields on the
wafer 22.
Exposure apparatus as provided herein are not limited to
microlithography apparatus for manufacturing microelectronic
devices. As a first alternative, for example, the exposure
apparatus can be a microlithography apparatus used for transferring
a pattern for a liquid-crystal display (LCD) onto a glass plate. As
a second alternative, the exposure apparatus can be a
microlithography apparatus used for manufacturing thin-film
magnetic heads. As a third alternative, the exposure apparatus can
be a proximity-microlithography apparatus used for exposing, for
example, a mask pattern. In this alternative, the mask and
substrate are placed in close proximity with each other, and
exposure is performed without having to use a projection-optical
system 20.
The principles set forth in the foregoing disclosure further
alternatively can be used with any of various other apparatus,
including (but not limited to) other microelectronic-processing
apparatus, machine tools, metal-cutting equipment, and inspection
apparatus.
In any of various exposure apparatus as described above, the source
16 (in the illumination-optical system 18) of illumination "light"
can be, for example, a g-line source (436 nm), an i-line source
(365 nm), a KrF excimer laser (248 nm), an ArF excimer laser (193
nm), or an F.sub.2 excimer laser (157 nm). Alternatively, the
source 16 can be of a charged particle beam such as an electron or
ion beam, or a source of X-rays (including "extreme ultraviolet"
radiation). If the source 16 produces an electron beam, then the
source can be a thermionic-emission type (e.g., lanthanum
hexaboride (LaB.sub.6) or tantalum (Ta)) of electron gun. If the
illumination "light" is an electron beam, then the pattern can be
transferred to the wafer 22 from the reticle 12 or directly to the
wafer 22 without using a reticle.
With respect to the projection-optical system 20, if the
illumination light comprises far-ultraviolet radiation, then the
constituent lenses are made of UV-transmissive materials such as
quartz and fluorite that readily transmit ultraviolet radiation. If
the illumination light is produced by an F.sub.2 excimer laser or
EUV source, then the lenses of the projection-optical system 20 can
be either refractive or catadioptric, and the reticle 12 desirably
is a reflective type. If the illumination "light" is an electron
beam (as a representative charged particle beam), then the
projection-optical system 20 typically comprises various
charged-particle-beam optics such as electron lenses and
deflectors, and the optical path should be in a suitable vacuum. If
the illumination light is in the vacuum ultraviolet (VUV) range
(less than 200 nm), then the projection-optical system 20 can have
a catadioptric configuration with beam splitter and concave mirror,
as disclosed for example in U.S. Pat. Nos. 5,668,672 and 5,835,275,
incorporated herein by reference. The projection-optical system 20
also can have a reflecting-refracting configuration including a
concave mirror but not a beam splitter, as disclosed in U.S. Pat.
Nos. 5,689,377 and 5,892,117, incorporated herein by reference.
Either or both the reticle stage 14 and wafer stage 24 can include
respective linear motors for achieving the motions of the reticle
12 and wafer 22, respectively, in the X-axis and Y-axis directions.
The linear motors can be air-levitation types (employing air
bearings) or magnetic-levitation types (employing bearings based on
the Lorentz force or a reactance force). Either or both stages 14,
24 can be configured to move along a respective guide or
alternatively can be guideless. See U.S. Pat. Nos. 5,623,853 and
5,528,118, incorporated herein by reference.
Further alternatively, either or both stages 14, 24 can be driven
by a planar motor that drives the respective stage by
electromagnetic force generated by a magnet unit having
two-dimensionally arranged magnets and an armature-coil unit having
two-dimensionally arranged coils in facing positions. With such a
drive system, either the magnet unit or the armature-coil unit is
connected to the respective stage and the other unit is mounted on
a moving-plane side of the respective stage.
Movement of a stage 14, 24 as described herein can generate
reaction forces that can affect the performance of the exposure
apparatus. Reaction forces generated by motion of the wafer stage
24 can be shunted to the floor (ground) using a frame member as
described, e.g., in U.S. Pat. No. 5,528,118, incorporated herein by
reference. Reaction forces generated by motion of the reticle stage
14 can be shunted to the floor (ground) using a frame member as
described in U.S. Pat. No. 5,874,820, incorporated herein by
reference.
An exposure apparatus such as any of the various types described
above can be constructed by assembling together the various
subsystems, including any of the elements listed in the appended
claims, in a manner ensuring that the prescribed mechanical
accuracy, electrical accuracy, and optical accuracy are obtained
and maintained. For example, to maintain the various accuracy
specifications, before and after assembly, optical system
components and assemblies are adjusted as required to achieve
maximal optical accuracy. Similarly, mechanical and electrical
systems are adjusted as required to achieve maximal respective
accuracies. Assembling the various subsystems into an exposure
apparatus requires the making of mechanical interfaces,
electrical-circuit wiring connections, and pneumatic plumbing
connections as required between the various subsystems. Typically,
constituent subsystems are assembled prior to assembling the
subsystems into an exposure apparatus. After assembly of the
apparatus, system adjustments are made as required for achieving
overall system specifications in accuracy, etc. Assembly at the
subsystem and system levels desirably is performed in a clean room
where temperature and humidity are controlled.
Seventh Representative Embodiment
Any of various microelectronic devices and displays can be
fabricated using an apparatus as described above in the sixth
representative embodiment. An exemplary process is depicted in FIG.
16. In step 301, the function and performance characteristics of
the subject device are designed. Next, in step 302, a mask
(reticle) defining a corresponding pattern is designed according to
the specifications established in the preceding step. In a parallel
step 303 to step 302, a wafer or other suitable substrate is made.
In step 304 the mask pattern designed in step 302 is exposed onto
the wafer using an exposure apparatus as described herein. In step
305 the microelectronic device is assembled; this typically
includes dicing, bonding, and packaging steps as well known in the
art. Finally, in step 306, the devices are inspected.
FIG. 17 is a flow chart of details of step 304, as applied to
manufacturing microelectronic devices. In step 311 (oxidation) the
surface of the wafer is oxidized. In step 312 ("CVD" or chemical
vapor deposition) an insulating film is formed on the wafer
surface. In step 313 (electrode formation) electrodes are formed on
the wafer by vapor deposition. In step 314 (ion implantation) ions
are implanted in the wafer. These steps 311-314 constitute the
"pre-process" steps for wafers during wafer processing; during
these steps selections are made as required according to processing
requirements.
Continuing further with FIG. 17, at each stage of wafer processing,
after the above-mentioned pre-process steps are completed, the
following "post-process" steps are executed. Initially, in step 315
(photoresist formation), a layer of a suitable resist is applied to
the wafer surface. Next, in step 316 (exposure), the
microlithography apparatus is used to transfer the circuit pattern
defined by the mask (reticle) to the wafer. In step 317
(developing) the exposed layer of resist on the wafer surface is
developed. In step 318 (etching), portions of the wafer surface not
protected by residual resist are removed by etching. In step 319
(photoresist removal) any resist remaining after completing the
etching step is removed.
Multiple circuit patterns are formed on the wafer surface by
repeating these pre-process and post-process steps as required.
Whereas the invention has been described in connection with
multiple representative embodiments and examples, it will be
understood that the invention is not limited to those embodiments
and examples. On the contrary, the invention is intended to
encompass all modifications, alternatives, and equivalents as may
be included within the spirit and scope of the invention, as
defined by the appended claims.
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